Silicon-based micro-electro-mechanical system (MEMS) microphones, also known as acoustic transducers, have been studied for more than 20 years. Because of their potential advantages in miniaturization, performance, reliability, environmental endurance, low cost, and mass production capability, the MEMS microphones are gaining ground over conventional microphones. Of all the silicon-based approaches, capacitive microphones are the most popular.
FIG. 1 illustrates a conventional MEMS microphone 2. Two polysilicon films (membranes) 4 and 6 are parallel to, and close to, each other. The illustrated openings in films 4 and 6 are small holes. Polysilicon film 4 is fixed by the structure, and hence is substantially unmovable. Polysilicon film 6 includes a center portion that can vibrate, and fixed end portions. In response to acoustic wave, polysilicon film 6 vibrates, and hence its distance from polysilicon film 4 also changes. As a result, the capacitance of the capacitor that has polysilicon films 4 and 6 as two capacitor plates also fluctuates in response to the acoustic wave. Such fluctuation in capacitance is picked up by electrode 8, which is connected to polysilicon film 6, and another electrode (not shown) that is connected to polysilicon film 4.
MEMS microphone 2 suffers from drawbacks. First, since there are two polysilicon films, the respective manufacturing cost and cycle time are relatively high. Second, since polysilicon films 4 and 6 are closely located to each other, if vapor causes the sticking of polysilicon film 4 to polysilicon film 6, capacitor 2 will not be able to function properly, and the electrical signal generated from the acoustic wave will be distorted. New MEMS microphones with reduced manufacturing cost and improved reliability are thus needed.